This invention generally relates to polyimide resins. More specifically, the invention relates to such resins, and articles comprising the same, useful in high temperature and/or dielectric applications.
Over the last decade, significant increases in capacitor reliability have been achieved through a combination of advanced manufacturing techniques and new materials. Greatly enhanced performance has been obtained particularly in so-called film capacitors. Film capacitors can be classified into three types based on the manufacturing technology, namely, film and foil capacitors, metallized film capacitors and mixed technology film capacitors.
Generally, metallized film capacitors consist of two metal electrodes separated by a layer of plastic film. The metallized plastic film is constructed by vacuum depositing metal film onto a layer of plastic film. This would offer compact capacitor structure, self-clearing capability, longer lifetime, and higher energy density. Some of the commonly used plastic films are polypropylene and polyetherimide films. The metal film layer is typically extremely thin, in the order of about 200-500 angstroms and is typically aluminum, zinc or alloys of the same. Compared to other types of capacitors, metallized film capacitors provide advantages in size, simplicity, and cost of manufacturing, and hence have been widely used in the power electronics industry.
While significant improvements have been made in metallized film capacitors, certain issues, such as thermal stability and reduced lifetime continue to present challenges to their widespread use. For example, polyimides, such as polyetherimides, made from dianhydrides and diamines have been used in the past for the manufacture of thin film capacitors, as have polypropylene, polycarbonate and certain polyesters. Although polyimides and polyetherimides have relatively high glass transition temperatures (217° C. and higher) the glass transition temperatures of polypropylene, polycarbonate and polyesters are lower (usually less than 150° C.) and can limit the continuous-use, working temperatures of articles made from these resins to about 120° C. or less. Further, the relatively low dielectric constant of each of these resins limits the stored energy capacitors made from them to about 1 Joule/cc.
New polymeric resins would thus desirably be provided, having higher dielectric constants, and capable of operating at temperatures in excess of 120° C. Such resins would be expected to be useful in a wide variety of articles, including thin film capacitors, and metalized thin film capacitors in particular.